A fuel cell stack is a new device for generating electricity and comprises a plurality of fuel cell units. Each fuel cell unit directly converts chemical energy produced by an electrochemical reaction of a fuel gas (or reaction gas) and an oxidizing gas into electrical energy. A fuel cell unit is similar to a general cell due to the fact that it comprises two electrodes and an electrolyte. However, a fuel cell unit is different from a general cell because the fuel gas and oxidizing gas are continuously supplied to the fuel cell unit.
Fuel cells can be classified into several different categories. For example, molten carbonate fuel cells are fuel cells which operate at high temperatures ranging from about 500.degree. C. to about 700.degree. C. Phosphate fuel cells are cells which operate at temperatures around 200.degree. C. Alkaline fuel cells are cells which operate from room temperature to about 100.degree. C. Finally, solid fuel cells are cells which operate at high temperatures of 1000.degree. C. or more.
Typical voltages produced by a single fuel cell unit range from 0.7 to 1.0 V under a 100-200 mA/cm.sup.2 current density. Therefore, power densities of 0.5 to 2 kW/m.sup.2 can be produced by a single fuel cell unit. Accordingly, in order to obtain higher voltages and currents, a fuel cell stack can be created by arranging a plurality of fuel cell units in series or parallel with each other.
Most fuel cell stacks comprise fuel cell units which are arranged in series and which have anodes and cathodes respectively connected to bipolar plates. The bipolar plates are made from a material which is conductive and resists corrosion in a strong oxidizing and reducing environment.
The operation of a fuel cell unit within a fuel cell stack will be described with reference to FIG. 1. As shown in the figure, a fuel cell unit comprises a matrix 1 made of a porous insulating material, an anode 2, and a cathode 3. In order to form a fuel cell stack, a plurality of fuel cell units are stacked on top of each other by interposing bipolar plates 4 between adjacent fuel cell units. When a reaction gas such as hydrogen gas is supplied to the anode 2 of one of the fuel cell units, the gas is transformed by a catalytic reaction into hydrogen ions and electrons. Subsequently, the hydrogens ion travel across the electrolyte to cathode 3.
In addition, an oxidizing gas such as oxygen gas is supplied to the cathode 3 of the fuel cell unit. As a result, the hydrogen ions, which traveled across the electrolyte, and the oxygen gas electrochemically react to produce water vapor and electricity.
In the case of a phosphoric acid fuel cell, the fuel cell units are stacked on top of each other and are separated by graphite bipolar plates. Consequently, a single large fuel cell stack can be created to produce a large amount of electrical power by supplying the stack with the required amount of reaction gas.
The manners in which the reaction gases can be supplied to the fuel cell stack are classified into two categories: the internal manifold supply method (FIG. 2) or the external manifold supply method (FIG. 3).
FIG. 2 illustrates the internal manifold supply method. As shown in the figure, hydrogen gas is supplied to the fuel cell stack via an inlet 21 and is distributed to the individual fuel cell units by an internal manifold 23 provided inside the stack. Subsequently, the gas passes over the anode 2 of each fuel cell unit via hydrogen gas channels 25, is collected by an internal manifold 24, and is discharged from the manifold 24 via an outlet 22. In addition, oxygen gas passes over the cathodes of each of the fuel cell units via oxygen gas channels 26. Each hydrogen gas channel 25 is separated from each oxygen gas channel 26 by the matrix and electrolytes 27 of each fuel cell unit.
FIG. 3 illustrates the external manifold supply method. As shown in the figure, hydrogen gas is supplied via an inlet 31 to an external manifold 33 located outside of the fuel cell stack. Subsequently, the external manifold 33 distributes the hydrogen gas to each fuel cell unit via the hydrogen gas channels 25, is collected by an external manifold 34, and is discharged from the manifold 34 via an outlet 32.
As indicated by the description above, the internal manifold supply method is safer than the external manifold supply method because the gas seals within the internal manifold supply system are easier to maintain and are more reliable. However, as the number of the fuel cell units increases in an internal manifold supply system, the difficulty of uniformly supplying the reaction gas to each fuel cell unit increases. Therefore, as the number of fuel cell units increase, the performance of the fuel cell stack decreases.
On the other hand, the external manifold supply system is advantageous over the internal manifold supply system because the reaction gas can be more uniformly distributed to each fuel cell unit. However, as the number of fuel cell units increase, the size of the fuel cell stack significantly increases. Furthermore, maintaining adequate gas seals around the fuel cell units becomes extremely difficult because the external manifold is provided outside the stack.
In addition, a fuel cell stack implementing an internal manifold supply system and a fuel cell stack implementing an external manifold supply system also have problems which are common to both devices. For example, neither system has the ability to selectively isolate a portion the fuel cell units within the stack. Thus, if a fuel cell unit malfunctions, the supply of reaction gas to the defective unit cannot be stopped. Consequently, the efficient operation of both of the fuel cell stacks significantly decreases if one of the fuel cell units fails.